Question

Nitrogen does not pentavalent give reason

Answer 1

Nitrogen can't be pentavalent in resonance structures because of arbitrary restrictions that say that nitrogen must always follow the octet rule, but phosphorus need not follow that rule.

But nitrogen can have oxidation states up to +5, and is pentavalent in nitric acid.

The “semipolar bond” in nitric acid (in which N+ is single-bonded to O-, making a double bond consisting of one covalent and one ionic bond) is just as much a double bond as the P=O double bond in phosphoric acid. Some chemistry drawing styles show nitrogen in nitric acid with two double bonds and one single bond.

Answer 2

Because it doesn't possess vacant d orbitals!